Sunday, March 24, 2019

Velleman K4040, take one

This is the first of three posts on Velleman K4040. Here are the links to the second and third posts.

My second amp was the big, hot and heavy K4040, built from a Velleman kit. The kit is not available anymore, but the amplifier still looks quite impressive:
 

Two quads of EL34 tubes, one per channel, in ultralinear connection provide 2x90W of power into either 8 or 4ohm load. The power and output transformers are large toroids:

The amplifier also has a nice bias adjustment system for the eight power tubes - behind the front panel, there is a line of LEDs showing the bias level of any tube, one at a time, and trimpots for adjusting.

Features:

  • Pure valve sound with high quality EL34 valves. 􏰄 High quality black and chrome housing
  • Chrome valve socket covers
  • Easy bias adjustment with LED indication
  • Removable bottom for easy access and service. 􏰄 High quality capacitors and components
  • Gold plated input and speaker terminals
  • Standby function
  • 􏰄Soft start circuit for power transformer

Specifications:

  • 􏰂2x90Wrms into 4 or 8ohm
  • Up to 2 x 15Wrms full class A
  • Bandwidth: 8Hz to 80KHz (-3dB/1W)
  • Harmonic distortion: 0.1% @ 1W/1KHz
  • Signal to noise ratio: >105dB (A weighted)
  • Input sensitivity: 1Vrms
  • 115VAC or 230VAC / 500VA max.

The schematic of the amplifier itself is quite conventional - two stages of triode amplification, followed by a cathodyne phase splitter, followed by a power stage with four power pentodes running in ultralinear mode with fixed bias:


The power supply includes many nice features, such as soft start, stand-by mode with input muting, and the above mentioned power tubes bias monitor with LED indication:

For reference, the toroidal output transformers of K4040 have approx. 1.8k Raa primary winding (15:1 turns ratio, anode-anode to 8ohm) with 33% screen (ultralinear) taps, and a 8ohm secondary with 4ohm tap (which is also used for feedback). The DC resistance of the primary is 41ohm anode to anode (20-21ohm B+ to each anode) and about 7ohm from B+ to each grid.

Contrary to some reviews, I found this amp sounding rather nice, particularly after I replaced the EL34 tubes in the output stage with separately purchased matched quads.

However, the sound did not quite match the price, so I modified this amplifier, leaving little of the original design.

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